An ion transfer assembly for directing ions from an atmospheric pressure ion source into an ion trap mass spectrometer with reduced random noise during the analysis of the transferred ions by the ion trap mass spectrometer.
Atmospheric pressure ion sources coupled to mass spectrometers by an ion transfer assembly often produce random noise spikes which can severely limit the signal-to-noise ratio in the mass spectra. These noise spikes are believed to be caused by charged particles or clusters ions which reach the detector region at random times. The abundance of the noise can be affected by several parameters related to the ion source including spray stability, involatile buffer concentration, solvent flow, and sampling configuration. This noise has been shown in U.S. Pat. No. 5,171,990 to be reduced in an ion transfer assembly by moving the capillary off-axis from the skimming electrode at a small cost in sensitivity but with a large increase in signal-to-noise ratio.
Ion trap mass spectrometers such as described in U.S. Pat. Nos. 4,540,884 and 4,736,101, and the various forms described in U.S. Pat. No. 5,420,425 have the advantage that the injection of ions from the ion source occurs at a different time than when the mass spectrum is taken and therefore allows rejection of the charged particles and ions during mass analysis. This allows the appropriate electric field for this rejection to be used during the time a mass analysis is being carried out. One approach that has been used is described in U.S. Pat. No. 5,750,993 and involves simply putting a lens (one which resides at lower pressures) to a high repelling potential at the appropriate time to block charged particles and ions from entering the ion trap mass spectrometer during mass analysis. However, large voltages ( greater than 300V) are necessary for this method and high energy noise particles still may penetrate the blocking potential. The invention described here utilizes a transverse dipole field along the entire length of a RF multi-pole ion guide to deflect the noise particles and prevent them from entering the ion trap. This method requires less voltage and is more effective in stopping the noise particles and ions from entering the ion trap during mass analysis.
It is an object of the present invention to provide an ion transfer assembly for directing ions from an atmospheric pressure ionization source of an ion trap mass spectrometer with reduced random noise and to a method of operation of the ion transfer assembly.
It is another object of the present invention to provide an ion transfer assembly employing multi-rod ion guides and means for applying appropriate RF and DC voltages to the rods which allows efficient transmission of ions to an ion trap while being able to reject random noise during mass analysis.
The foregoing and other objects of the invention are achieved by an ion transfer assembly which includes multi-rod ion guides for transferring ions from an atmospheric pressure ion source to an ion trap mass spectrometer, including means for applying RF and DC voltages to said rods to transfer ions into the ion trap for analysis by the mass spectrometer, and means for applying a DC voltage to said rods to create a dipolar field transverse to the ion path axis (with or without RF voltages), while the ions are analyzed by the mass spectrometer to minimize noise introduced by charged particles, desolvated charged droplets and ions from the atmospheric pressure ionization source by deflecting the particles and ions.